The invention relates generally to power amplifiers, and in particular to linearizing the input/output transfer function for amplifiers, particularly high power class AB power amplifiers.
High power, broad band power amplifiers are well known. These amplifiers may operate in a feed forward configuration, or may have other forms of linearization which are required when the main power amplifier operates, for example, as a class AB amplifier. Although class A amplifiers usually produce less distortion than class AB amplifiers, class A amplifiers are also less efficient than class AB amplifiers. Thus, in order to retain the advantages of efficiency while minimizing distortion, class AB amplifier configurations have been developed which implement various forms of error or distortion correction.
One form of error correction attempts to distort the input signal in a manner which compensates for the distortions of a class AB amplifier. Thus, a predistortion circuit can be provided with various manual adjustments to produce a distortion signal from the original signal, so that when the distortion signal is combined with the input signal, and the combination is input to the power amplifier, operating for example, as a class AB amplifier, the output is substantially a linear amplification of the original input signal to the amplifier arrangement.
Such predistortion circuities typically employ a low power amplifier, preferably having the same general distortion characteristics as the main amplifier, so that its output, properly processed, can be used to obtain the necessary distortion components required to be combined with the input signal to the predistortion circuitry to generate a predistorted input to the main amplifier. Such configurations operate to substantially reduce the intermodulation frequency distortions produced by a class AB amplifier when the variable elements of the predistortion circuitry are properly adjusted.
However, even in properly adjusted amplifier arrangements using predistortion, a certain amount of instability, that is, drift in the operating point gain and/or phase, can be observed. Thus, a microprocessor can adjust the critical parameters of a predistortion circuit to minimize the distortion components, but has the limitation that as the predistortion circuit is adjusted, it unbalances the cancellation loop in the feed-forward configuration. This requires the microprocessor to re-null the signal cancellation loop before a decision can be made as to whether an improvement has actually been achieved. This is a very time consuming process, and if the input signals are of a type which are in a constant state of change, for example, turned on and off, the loops will become "confused" and not yield an adaptive improvement. Thus, the prior art adaptive predistortion techniques such as that disclosed in U.S. application Ser. No. 09/057,332, filed Apr. 8, 1998, and entitled, DYNAMIC PREDISTORTION COMPENSATION FOR A POWER AMPLIFIER, the contents of which are incorporated herein, in their entirety, by reference, works well for steady state signals but fails to give the results sought if operated in a transient environment. That transient environment exists for certain types of multi-channel operation, with the analog cellular system being a good example. In that system, carriers are continually being switched on and off.